This invention relates to paint curtain machines for coating a substrate with a fluid paint and more particularly to such machines wherein the paint is a dielectric ceramic precursor and the paint machine is adapted for use in the manufacture of monolithic ceramic capacitors.
Paint curtain machines are known wherein paint is discharged from a paint reservoir to form a continuously falling sheet or curtain of paint. A substrate material is passed through the falling curtain and a layer of paint is deposited on the top surface of the substrate. The faster the substrate is passed through the curtain or the thinner the curtain itself, then the thinner the substrate coating becomes. Excess paint from the curtain is captured in a collector pan under the line of passage of the substrate and is pumped back up into the paint reservoir. The rate of pumping determines and is about proportional to the thickness of the paint curtain.
Such paint curtain machines are particularly well suited to a high speed process for continuously painting either a long continuous sheet substrate material, or for sequentially painting a series of discrete substrates that are carried through the curtain on a conveyor.
As an example of the former, a cloth or fabric is continuously passed through a curtain of opaque paint in the manufacture of window shades.
An example of the latter consists in the method of making monolithic ceramic capacitors wherein discrete substrates of porous paper or cardboard are repeatedly passed through a ceramic precursor paint with electrode films being deposited between some of the successive paint layers. The ceramic paint layers are heated to remove the solvent and the final stack of dried paint layers with electrodes buried therein is diced into individual capacitor units and fired to maturity. Such a process is described in U.S. Pat. No. 3,717,487, issued Feb. 20, 1973 and assigned to the same assignee.
The paint curtain formed in such machines is desirably thin but such thin falling curtains tend to draw together from the line of departure or discharge from the paint reservoir so that unless the discharge line is very long, the paint pours onto a substrate in a central stream. To prevent this drawing or gathering of the paint curtain, it has become conventional to hang either a bead chain or a straight bar from the reservoir in the curtain and near each end of the discharge line. These two bead chains or two hanging bars reduce the drawing together of the falling curtain although they themselves are pulled by the paint so that they hang with a bias toward the vertical center line of the curtain. These chains or bars are sometimes called curtain stretchers.
However, in the vicinity of the hanging chains or bars the curtain contains wrinkles that produce alternate stripes of thick and thin regions in the substrate paint coating which is particularly objectionable in a dielectric layer of a capacitor. Moreover, it becomes more difficult to form a paint curtain with a broad wrinkle-free central region as the paint pumping rate is further reduced to provide an even thinner curtain and thus a thinner dielectric coating for a fixed speed of passage of the substrates. Not only are wrinkles more pronounced in thinner curtains, but now the curtain is more easily "torn" when the machine is inadvertently bumped or vibrated or when there is a breeze in the ambient air. Such tears usually occur in the wrinkled region of the curtain where the curtain is alternatively abnormally thick and thin. The tears often become wide enough to extend 6 inches or more from the edge of the curtain toward the center or even to cause the curtain to break completely and degenerate into a random series of drops and streams.
As an alternative to thinning the paint curtain to obtain a thinner paint coating on the substrate, the speed of passage of the substrate through the curtain may be increased. This technique is limited by the tendency for defects of yet another kind to be formed in the substrate coating, the density of such coating defects increasing with increasing speed of the substrates. Even minor discontinuities in the substrate surface, such as those occurring at the to-be-buried electrode film edges of a monolithic ceramic capacitor under construction, cause the paint coating to skip in these regions when the substrate speed through the thin curtain is too high. Although the paint coating subsequently tends to level, thin or even bare regions in the coating still remain and at best become regions of low dielectric strength.
Thus, both techniques for obtaining a thinner paint coating are limited in practice so that highly uniform ceramic dielectric films less thin than about 0.002 inches cannot be made with high yields using paint curtain machines known heretofore.
It is therefore an object of this invention to provide an improved paint curtain machine that is capable of providing a thin substantially wrinkle free falling paint curtain.
It is a further object of this invention to provide a paint curtain machine that is capable of producing a thin falling paint curtain that is more stable and less subject to tearing due to wind or to bumps imparted to the machine.
It is yet a further object of this invention to provide a means and a method for depositing highly uniform dielectric films with high yields, such films having a thickness of about 0.001 inch or less.
It is still a further object of this invention to provide a method for making reliable thin ceramic films for use as the dielectric material in a monolithic ceramic capacitor.